1. Field of the Invention
The present invention relates to a liquid crystal display substrate, a method of manufacturing the same, and a liquid crystal display device having the same, and more particularly, it relates to such a substrate for transreflective liquid crystal display that can attain display in both a transmission mode and a reflection mode, a method of manufacturing the same, and a liquid crystal display device having the same.
2. Description of the Related Art
In recent years, liquid crystal devices are demanded to have higher performance. According to spread of mobile phones and mobile electronic devices, in particular, they are strongly demanded to attain low electric energy consumption and good usability out of doors. In order to attain low electric energy consumption and good usability out of doors, a reflection liquid crystal display device has been proposed, which has a pixel electrode having light reflection capability (a reflection electrode) and attains display by reflecting outside light to make a light source device unnecessary.
A thin film transistor (TFT) substrate of a reflection liquid crystal display device has a reflection electrode formed thereon with a metallic thin film having high light reflectivity. In the reflection liquid crystal display device, natural light incident thereon from the display screen side or light emitted by utilizing electricity is reflected by the reflection electrode on the TFT substrate, and the reflected light is used as a light source for liquid crystal display. The reflection electrode has an uneven surface. The uneven surface of the reflection electrode can be obtained by previously forming a light-sensitive resin layer having an uneven surface as an underlayer of the reflection electrode. The light incident from the display screen side is diffusely reflected by the uneven surface of the reflection electrode to obtain high luminance and a large viewing angle.
In the reflection liquid crystal display devices disclosed in JP-A-2002-221716 and JP-A-2002-296585, for example, a surface (an upper layer portion) of an overcoat layer formed with a resin material is applied to predetermined energy to make the upper layer portion be relatively cured in comparison to a lower layer portion, and then the overcoat layer is subjected to a heat treatment at a temperature equal to or higher than the heat curing point thereof, whereby wrinkled unevenness is formed on the surface of the overcoat layer.
A transreflective liquid crystal display device is also proposed, which can attain display in a transmission mode in addition to display in a reflection mode as similar to the reflection liquid crystal display device. In the transreflective liquid crystal display device, a transmission area having a transparent electrode formed with a light transmission material and a reflection area having a reflection electrode formed with a light reflection material are formed on each of pixel areas. The reflection electrode of the transreflective liquid crystal display device is formed on a resin layer having an uneven surface, as similar to the reflection liquid crystal display device. The transreflective liquid crystal display device referred herein includes a slightly transmission liquid crystal display device, which has an increased proportion of the reflection area in pixel areas to improve display luminance in a reflection mode, and a slightly reflection liquid crystal display device, which has an increased proportion of the transmission area in pixel areas to improve display luminance in a transmission mode.
FIG. 27A is a plan view showing a constitution of a TFT substrate of a conventional transreflective liquid crystal display device. FIG. 27B is a cross sectional view showing the TFT substrate shown in FIG. 27A on line X-X. As shown in FIGS. 27A and 27B, a glass substrate 110 of the TFT substrate 102 has a plurality of gate bus lines 112 extending in parallel to each other in the landscape direction in FIG. 27A (provided that only one of them is shown in FIGS. 27A and 27B).
An insulating film 130 is formed on the gate bus lines 112 on the entire surface of the substrate (which is sometimes referred to as a gate insulating film, depending on the position where the film is formed). A plurality of drain bus lines 114 are formed extending in parallel to each other in the portrait direction in FIG. 27A as intersecting the gate bus lines 112 with the insulating film 130 intervening therebetween (provided that only two of the drain bus lines 114 are shown in FIG. 27A). TFTs 120 are formed in the vicinities of positions where the gate bus lines 112 and the drain bus lines 114 are intersected each other.
The TFT 120 has an active semiconductor layer 128 formed with an a-Si layer on the insulating film 130. A channel protective film 123 is formed on the active semiconductor layer 128. The gate bus line 112 in an area immediately beneath the channel protective film 123 is configured to function as a gate electrode of the TFT 120. The channel protective film 123 has thereon a drain electrode 121 drawn from the adjacent drain bus line 114 and a source electrode 122 disposed to face the drain electrode 121 through a predetermined gap.
A protective film 132 is formed on the TFT 120 on the entire surface of the substrate. A wrinkled resin layer 134 having wrinkled unevenness on the surface thereof is formed on the protective film 132 in a reflection area of each of the pixel areas. A reflection electrode 117 is formed on the wrinkled resin layer 134. The reflection electrode 117 has a wrinkled uneven surface following the surface of the wrinkled resin layer 134. The reflection electrode 117 and the wrinkled resin layer 134 are formed to cover the TFT 120. Separately, a transparent electrode 116 is formed on the protective film 132 in a transmission area of each of the pixel areas. One pixel is constituted with the reflection area and the transmission area positioned on the adjacent upper side of the reflection electrode in FIG. 27A. The reflection electrode 117 and the transparent electrode 116 in the same pixel are electrically connected to each other. The transparent electrode 116 is electrically connected through a contact hole 124 to a source electrode 122 of a TFT 120 formed as an underlayer of a reflection electrode 117 of a pixel positioned on the adjacent upper side in FIG. 27A.
A storage capacitor bus line 118 is formed on the glass substrate 110 in parallel to the gate bus line 112 as extending in the landscape direction in FIG. 27A. The storage capacitor bus line 118 functions as one electrode of a storage capacitor. A storage capacitor electrode 119 is formed on the storage capacitor bus line 118 through the insulating film 130. The storage capacitor electrode 119 is electrically connected to the source electrode 122 and functions as the other electrode of the storage capacitor. A light leakage preventing film 140 is also formed on the glass substrate 110 in parallel to the gate bus line 112 and the storage capacitor bus line 118 in the landscape direction in FIG. 27A. The light leakage preventing film 140 is disposed to shielding the vicinity of the boundary between the reflection area and the transmission area from light, so as to prevent leakage of light caused by alignment failure of the liquid crystal in the vicinity of the boundary between the areas.
The wrinkled resin layer 134 in the TFT substrate 120 shown in FIGS. 27A and 27B is formed by the following procedures. A positive light-sensitive resin is coated on a whole surface of a glass substrate having TFTs and the like formed thereon to form a resin layer. The glass substrate is placed on an exposing stage in an exposing apparatus, and the resin layer is exposed through a photomask that shields areas to be reflection areas from light. By this, the resin layer is exposed on areas other than the reflection areas. Subsequently, the resin layer is developed to remove the resin layer in the exposed area by dissolving in a developer solution, whereby the resin layer in the non-exposed reflection areas remains as not dissolved in the developer solution. The surface of the remaining resin layer is irradiated with UV light to cure the upper layer portion of the resin layer. Subsequently, the resin layer is subjected to a heat treatment at a temperature equal to or higher than the heat curing point thereof, so as to form a wrinkled resin layer having wrinkled unevenness on the surface thereof.
In the step of exposing the resin layer, however, the light reflected by the surface of the exposing stage is also incident on the resin layer in the reflection areas. Accordingly, the resin layer in the reflection areas is exposed and cured to such an extent that it is not dissolved in the developer solution. In general, the surface of the exposing stage has grooves formed thereon. Therefore, the intensity of the light incident on the resin layer in the reflection areas varies depending on the presence and absence of the grooves on the surface of the exposing stage, and thus, the extent of curing of the resin layer varies depending on the positions of the grooves. Accordingly, uniform wrinkled unevenness cannot be formed on the surface of the resin layer in the subsequent step to fluctuate the shape of wrinkled unevenness corresponding to the positions of the grooves on the surface of the exposing stage. Consequently, a transreflective liquid crystal display device thus manufactured has such a problem that display ununiformity corresponding to the positions of the grooves on the surface of the exposing stage is viewed upon display in a reflection mode, so as to fail to obtain an intended reflectivity and good reflection uniformity.